Due to increased interest in reducing the fuel consumption and exhaust pollution, while satisfying the customer demands for better performance, the automotive industry has become more interested in research and development of hybrid electric vehicles. Powertrains for such vehicles have advantages over conventional internal combustion engine propelled powertrains including reduced fuel consumption and exhaust emissions, while providing the same range of driving (not limited by batteries as in the case of pure electric vehicles).
The concept of the hybrid powertrain is not novel. The conceptual background is documented in several patents. Some of the recent ones are U.S. Pat. No. 5,656,921 granted to Farrall in 1997 and U.S. Pat. No. 5,343,970 granted to A. J. Severinsky in 1994.
In his U.S. Pat. No. 5,343,970, Sep. 6, 1994 Severinsky gives an extensive overview of different hybrid configurations. He provides specific reasons why the configuration he proposes provides advantages in comparison to all other hybrid vehicle configurations. He considers a parallel hybrid vehicle without a multiple-ratio transmission. Instead, a controllable torque transfer unit of fixed gear ratio is employed. Both an SI engine and an electric motor are on the same side of the torque transfer unit.
Severinsky describes the modes of operation of the hybrid vehicle: i.e., (i) operating the electric motor only at low speeds or in traffic, using the power stored in batteries; (ii) operating during heavy power demands when both engine and the motor provide the torque to the wheels; and (iii) operating steady state when the internal combustion engine alone drives the vehicle.
Farrall (U.S. Pat. No. 5,656,921; Aug. 12, 1997) also considers a parallel hybrid with both power sources on the same side of the transmission. It employs fuzzy logic rules to adjust the entries in the tables determining the power splitting between the SI engine and the electric motor. The performance measure used to adjust the entries is given by the weighted ratio between the battery current and fuel flow rate.
These prior art patents assume (implicitly) that the powertrain has a spark ignition engine which operates at fixed air-to-fuel ratio. The issues related to control of SI engines relate to controlling the air flow past the throttle. In most of the operating range for a given throttle opening a constant fuel flow is injected into the engine corresponding to the constant flow of air (in g/s) past the throttle. On the other hand, with a diesel engine cycle fuel can be directly controlled, so the issues related to controlling a diesel engine are unrelated to the issues of air management in spark ignited engines.
The '970 and '921 patents do not discuss consistent pedal feel in parallel hybrid vehicles nor do they describe a means for achieving it. The patents describe an SI (spark ignited) engine, not a direct injection diesel engine. They assume a throttle as an air control device. Finally, the '970 and '921 patents consider a different configuration of a parallel hybrid without transmission shifting considerations.